marked as duplicate by cegaton cyclesUsers with the cycles badge can single-handedly close cycles questions as duplicates and reopen them as needed.Apr 7 '18 at 20:37

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$\begingroup$Unfortunately you cropped your first image too tight. Do you have lights that are disabled for preview (eye icon in the outliner) and enabled for rendering (camera icon)? Is the Film exposure setting set to 1?$\endgroup$
– cegatonApr 6 '18 at 14:24

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$\begingroup$What do the nodes in the compositor look like?$\endgroup$
– cegatonApr 6 '18 at 16:00